Driving pulley for a continuously variable transmission

ABSTRACT

The driving pulley comprises two centrifugal mechanisms, namely a positive assembly and a negative assembly. Both assemblies comprise a respective set of flyweights subjected to the centrifugal force when the driving pulley rotates. The positive assembly is used as a conventional speed governor that moves one of the two flanges of the driving pulley towards the other in order to increase its winding diameter when the rotation speed increases. The negative assembly is used to apply an opposite force on the positive assembly when the rotation speed is above a threshold value so as to defer the upshift of the CVT to a higher ratio under the action of the positive assembly. This allows to maintain, for instance, a high rotation speed during an intense acceleration and a lower rotation speed when cruising at low vehicle speeds.

CROSS REFERENCE TO RELATED APPLICATION

This application is a US National Phase of International PatentApplication serial No. PCT/CA02/00314, filed Mar. 6, 2002 which claimsthe priority of U.S. Patent Application Ser. No. 60/273,643, filed Mar.7, 2001.

BACKGROUND OF THE INVENTION

A continuously variable transmission (CVT) is a mechanical device inwhich the torque transmission ratio is infinitely variable over theworking range, by contrast to a conventional transmission in which thereis a limited number of selectable torque transmission ratios. A CVTautomatically changes the ratio as required by load and speedconditions, providing an increased torque under high loads at low speedsand yet controlling the rotation speed of the motor as the vehicleaccelerates. It is commonly used in a wide range of vehicles, such assmall cars or trucks, snowmobiles, golf carts, all-terrain vehicles(ATV) and scooters. A CVT is usually coupled to a motor, such as aninternal combustion engine or an electric motor.

A conventional CVT comprises a driving pulley mechanically connected tothe motor, a driven pulley mechanically connected to wheels or a track,and a trapezoidal drivebelt transmitting torque between the drivingpulley and the driven pulley. The sides of the drivebelt are, on eachpulley, gripped between two opposite flanges that are coaxially mountedaround a main shaft. One of the flanges is axially movable withreference to the other. Each flange is directly or indirectly in atorque-transmitting engagement with the corresponding main shaft.

Initially, such as when the vehicle is stopped or at low speeds, thewinding diameter of the driving pulley is minimum and the windingdiameter of the driven pulley is maximum. This is referred to as theminimum ratio since there is the minimum number of rotations or fractionof rotation of the driven pulley for each complete rotation of thedriving pulley.

The driving pulley generally comprises a centrifugal mechanism that isprovided to increase the ratio when its rotation speed increases. To doso, the centrifugal mechanism is able to apply a force on the movableflange of the driving pulley to move it closer to the fixed flange, thusurging the drivebelt to wind on a larger diameter around the drivingpulley. At the same time, the shift in the position of the drivebelttowards the driving pulley urges the movable flange of the driven pulleyaway from the fixed flange thereof.

The driven pulley of a CVT is torque-sensitive. This allows the drivenpulley to counterbalance the force generated by the centrifugalmechanism of the driving pulley so as to allow the motor speed to riseto an optimum level before the CVT starts upshifting during anacceleration. The driven pulley also allows the CVT to downshift if theload increases. Accordingly, the driven pulley comprises a cam systemurging the movable flange to move towards the fixed flange of the drivenpulley when the torque increases, thereby pulling back on the drivebeltand fighting the force generated by the centrifugal mechanism of thedriving pulley. A conventional cam system comprises a cam plate having aplurality of symmetrically-disposed inclined cam surfaces on whichrespective cam followers are engaged. The cam followers are generallyslider buttons or rollers. The cam plate or the set of cam followers ismounted at the back side of the fixed flange and the other of them isusually rigidly connected to the main shaft.

In use, the movable parts of the CVT constantly seek to rearranged theirposition until an equilibrium is reached or until they reach the maximumratio. The ratio at which the CVT stabilizes is an equilibrium betweenthe forces on the drivebelt applied by the driving and the drivenpulley. At the maximum rotation speed, the ratio is maximum as there isthe maximum number of rotations or fraction of rotation of the drivenpulley for each complete rotation of the driving pulley. Then, when therotation speed of the motor decreases, the force generated by thecentrifugal mechanism decreases as well. Return springs located in thedriving and in the driven pulley allow the corresponding movable flangesto move back to or near their original low ratio position.

A conventional centrifugal mechanism of a driving pulley generallycomprises a set of centrifugal flyweights pushing their way through apair of opposite inclined ramps converging towards the periphery of thedriving pulley. Each of these flyweights are subjected to a centrifugalforce F as in the following equation:F=m rω² sin θwhere m is the mass of the flyweight, r is the radius from the center ofthe main shaft, ω is the rotation speed and θ is the angle of the rampswith reference to the main shaft. As one can see from the equation, theforce is a function of the square of the rotation speed, which meansthat the centrifugal force increases more rapidly that the proportionalincrease in the rotation speed itself. Also, the flyweights are movedaway from the center of the main shaft when the centrifugal forceincreases, which in turn also increases the force since the latterdepends on the radius r. It follows that the centrifugal system of thedriving pulley soon becomes proportionally stronger than the cam systemof the driven pulley, thereby shifting the position of the drivebelttowards the driving pulley. As a result, a conventional CVT tends toupshifts too early towards the maximum ratio when the rotation speed ofthe driving pulley increases. This is partially kept under control bychanging the angle of the ramps in function of the position of theflyweights, thus in function of the ratio. The angle of the ramps withreference to the axis of rotation is smaller at a higher ratio.

One of the drawbacks of a conventional driving pulley is thus the lackof direct control on the force generated by the centrifugal mechanism.Changing parameters such as the mass of the flyweights or the profile ofthe ramps to allow a higher motor speed during an acceleration is notalways a suitable or possible solution due to the impacts it has on theoverall behavior of the CVT. For instance, if the CVT is designed toallow a low rotation speed of the motor at a moderate cruising speed toreduce fuel consumption and noise, then during an intense acceleration,the rotation speed of the motor will most probably be too low.Conversely, if the CVT is designed for allowing a high rotation speed ofthe motor during an intense acceleration to obtain a maximum powertherefrom, then the rotation speed is likely to be too high at moderatecruising speed.

SUMMARY OF THE INVENTION

The present invention allows providing additional control over thetransmission shift pattern of a CVT so as to reduce the force generatedby the centrifugal system of the driving pulley when certain conditionsare met. Accordingly, the position of the driving pulley is normallycontrolled in a conventional way by a first set of flyweights, which arepart of an assembly referred to as the “positive assembly”. Then,beginning from a predetermined rotation speed, a second set offlyweights, which are part of an assembly referred to as the “negativeassembly”, will start coming into action. The basic purpose of thenegative assembly is to generate an axial force that is opposite the onegenerated by the positive assembly on the second flange. However, thatopposite force is not substantially active unless there is an engagementbetween the negative assembly and the positive assembly. The mass of theflyweights, the angles of the ramps, the presence and length ofstoppers, the rates and preload of the springs as well as all otherparameters are taken in account in the design so that the engagementbetween the negative and the positive assemblies is only happening ifproper conditions are met.

More particularly, the present invention provides a driving pulley for acontinuously variable transmission, the driving pulley being coaxiallymountable around a main shaft and rotatable at a variable rotationspeed, the driving pulley comprising:

-   -   a first flange having opposite first and second sides, the first        side being provided with a conical wall;    -   a second flange coaxial with the first flange and having a        conical wall facing the conical wall of the first flange to form        a drivebelt-receiving groove around which a drivebelt is wound,        the second flange being axially movable with reference to the        first flange;    -   first means for connecting the first flange to the main shaft in        a torque-transmitting engagement;    -   second means for connecting the second flange to the main shaft        in a torque-transmitting engagement;    -   a positive assembly comprising:        -   a positive carriage coaxial with the first flange and            rigidly connected to the second flange;        -   third means for connecting the positive carriage to the main            shaft in a torque-transmitting engagement;        -   at least two symmetrically-disposed pairs of            radially-converging and mutually-opposite first ramps, each            pair having one ramp connected to the positive carriage and            another ramp connected to the second side of the first            flange; and        -   radially-movable flyweights, each set between a            corresponding pair of first ramps;    -   fourth means for generating a return force urging the second        flange to move away from the first flange;    -   a negative assembly comprising:        -   a negative carriage coaxial and axially movable with            reference to the first flange, the negative carriage being            configured and disposed to be in engagement with the            positive carriage;        -   fifth means for connecting the negative carriage to the main            shaft in a torque-transmitting engagement;        -   at least two symmetrically-disposed pairs of            radially-converging and mutually-opposite second ramps, each            pair having one ramp connected to the negative carriage and            another ramp connected to an end plate fixed with reference            to the first flange;        -   sixth means for connecting the end plate to the main shaft            in a torque-transmitting engagement; and        -   radially-movable flyweights, each set between a            corresponding pair of second ramps; and    -   seventh means for generating a return force urging the negative        carriage away from the first flange.

BRIEF DESCRIPTION OF THE DRAWINGS

A non-restrictive description of a preferred embodiment will now begiven with reference to the appended figures, in which:

FIG. 1 is a longitudinal cross-sectional view of a driving pulleyaccording to a preferred embodiment of the present invention, showingtwo possible positions of the negative assembly when the driving pulleyis at a low ratio position.

FIG. 2 is a longitudinal cross-sectional view similar to FIG. 1, showingtwo possible positions of the negative assembly when the driving pulleyis at a high ratio position.

FIG. 3 is a radial cross-sectional view taken along line III—III in FIG.1.

FIG. 4 is a longitudinal cross-sectional view taken along line IV—IV inFIG. 2.

FIG. 5 is a radial longitudinal cross-sectional view taken along lineV—V in FIG. 4.

FIG. 6 is a graph showing three typical experimental curves (1,2,3) ofthe rotation speed of the driving pulley in function of the speed of thevehicle during a sustained and intense acceleration from a low speed anda typical curve (5) for a sustained and intense acceleration from amoderate speed (4).

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The following is a list of the reference numerals, along with the namesof the corresponding components, that are used in the appended figuresand in the description.

-   10 driving pulley-   12 belt-receiving groove-   14 trapezoidal drivebelt-   16 main shaft-   18 hollow drum-   20 end member (of the drum)-   22 cylindrical body (of the drum)-   24 connector-   28 first flange-   30 conical wall (of the first flange)-   36 second flange-   38 conical wall (of the second flange)-   40 sleeve (for the second flange)-   42 bushings (for the second flange)-   44 opening (in the first flange)-   46 bushing (in the opening of the first flange)-   50 positive assembly-   52 positive carriage-   54 bushings (of the positive carriage)-   56 cam followers (of the positive assembly)-   58 spindles (of the positive assembly)-   60 slots (in the drum)-   62 flyweights (of the positive assembly)-   64 first ramps (of the positive assembly)-   66 second ramps (of the positive assembly)-   70 spring (of the positive assembly)-   72 spring (of the negative assembly)-   74 intermediary part (of the cylindrical body)-   78 stoppers-   80 negative assembly-   82 negative carriage-   84 bushings (of the negative carriage)-   86 cam followers (of the negative assembly)-   88 spindles (of the negative assembly)-   90 slots (in the drum)-   92 flyweights (of the negative assembly)-   94 first ramps (of the negative assembly)-   96 second ramps (of the negative assembly)

The driving pulley (10) is primarily designed to be used in acontinuously variable transmission (CVT) of a vehicle, such as a smallcar or truck, a snowmobile, a golf cart, an all-terrain vehicle (ATV) ora scooter. However, it is possible to find other applications or otherenvironments where the driving pulley (10) may be advantageously used,such as in fixed commercial or industrial machines.

FIGS. 1 to 5 show a driving pulley (10) according to a possible andpreferred embodiment of the present invention. It should be noted thatthe parts shown in FIGS. 2 to 5 and which are not referred to,correspond to the same components than those shown in FIG. 1. Otherembodiments could also be devised within the scope of the presentinvention.

The driving pulley (10) is coaxially mounted around a main shaft (16)that is to be mechanically coupled to the output shaft of a motor (notshown), for instance an internal combustion engine of a motor vehicle.The main shaft (16) can be provided as a part of driving pulley (10) orbe an extension of an existing shaft around which the driving pulley(10) is mounted. The advantage of having a main shaft (16) as a part ofthe driving pulley (10) is that the latter can be pre-mounted anddirectly installed in the vehicle.

It should be noted that the term “coaxial” used in the description andthe claims only means that the corresponding elements have a commonmedial axis and does not mean that the elements have necessarily acircular cross section. Also, because the driving pulley (10) is to bedriven into high rotation speeds, all parts are balanced with referenceto the main shaft (16), as apparent to a person skilled in the art.

The driving pulley (10) comprises a first flange (28) and a secondflange (36), both facing each other and having opposite conical walls(30,38) defining between them a drivebelt-receiving groove (12). Atrapezoidal drivebelt (14) is wound in an arc around a large portion ofthe conical walls (30,38). About half the torque transmitted by thedrivebelt (14) goes through the first flange (28) while the other halfgoes through the second flange (36).

The first flange (28) is preferably supported by means of a hollow drum(18). The drum (18) comprises a radially-extending end member (20) and acylindrical body (22). The cylindrical body (22) is attached to theperiphery of the end member (20). The end member (20) is in atorque-transmitting engagement with the main shaft (16). To do so, theend member (20) can be rigidly connected to the main shaft (16) by anappropriate means, such as a spline or cone which is press fitted in acorresponding counter part. It can also be connected by fasteners,welding, press fitting, etc.

The drum (18) forms a housing enclosing and protecting most other partsof the driving pulley (10). It should be noted at this point that it ispossible to construct the drum (18) differently than what is shown inFIGS. 1 and 2. For instance, the end member (20) and the cylindricalbody (22) may be divided in space-apart strips (not shown) or be in theform of a rigid mesh (not shown).

The second flange (36) is preferably supported around the main shaft(16) by means of an elongated sleeve (40). The sleeve (40) is coaxiallymounted around the main shaft (16) and is free to slide with referenceto the main shaft (16). Bushings (42) are used to separate the sleeve(40) from the main shaft (16) and facilitate the sliding movement. Thesleeve (40) extends through a central opening (44) provided in themiddle of the first flange (28). The opening (44) encloses a bushing(46) and the outer surface of the sleeve (40) is in a sliding engagementwith the interior surface of the bushing (46). The bushings (42) canalso be replaced by a linear bearing (not shown) or the like.

FIGS. 1 and 2 further show a positive assembly (50) which governs therelative distance between the first flange (28) and the second flange(36) in function of the rotation speed of the driving pulley (10). Thedistance between the flanges (28, 36) is also a function of theresulting axial force created by the drivebelt (14) on their conicalwalls (30, 38). The positive assembly (50) comprises a positive assemblycarriage (52) coaxially and slidably mounted around the main shaft (16),preferably by means of bushings (54). The positive assembly carriage(52) is preferably rigidly connected to the sleeve (40) and supported byit.

As best shown in FIG. 3, the positive assembly carriage (52) isoperatively connected to the drum (18) by means of a plurality of camfollowers (56), which are symmetrically disposed with reference to themain shaft (16). The cam followers (56) are preferably rollers and threein number. Alternatively, the cam followers (56) can be slider buttons(not shown). Each roller (56) is preferably mounted around a bushing ora bearing. Each roller (56) is coaxially located around a respectiveradially-extending spindle (58) and is guided by alongitudinally-extending slot (60) located in the drum (18). The slots(60) have a width slightly larger than the outer diameter of the rollers(56). The rollers (56) are then freely longitudinally movable inside thecorresponding slot (60) and the length of the slots (60) substantiallycorresponds to the displacement of the second flange (36).

In use, the portion of the torque from the motor transmitted by thesecond flange (36) goes through the main shaft (16), the drum (18), thepositive assembly carriage (52) by means of the rollers (56) and theirslots (60), the sleeve (40) and then finally reaches the second flange(36). The torque can also be transmitted in the other direction, forexample during a deceleration. Alternatively, it is possible to deviseother ways of achieving the transmission of the torque, one being theuse of a linear bearing (not shown) between the sleeve (40) and the mainshaft (16).

As shown in FIGS. 1 and 2, the positive assembly (50) comprises aplurality of flyweights (62) symmetrically disposed with reference tothe main shaft (16). There are preferably three flyweights (62). Eachflyweight (62) is located between a corresponding pair of flyweightramps (64, 66). The number of flyweights (62) is equal to the number ofpairs of flyweight ramps (64, 66). Both ramps (64, 66) of each pair areradially converging with reference to the main shaft (16). In theillustrated embodiment, the first ramps (64) are provided on thepositive assembly carriage (52) and the second ramps (66) are integralwith the backside of the first flange (28).

As shown in FIG. 4, each flyweight (62) is preferably constructed inthree parts, namely a central cylindrical part and two identicalcylindrical side parts. The central part does not have the same diameterthat the two side parts. A bushing or bearing (not shown) allows adistinct rotation of the central part with reference to the two sideparts. One of the ramps (64, 66) is provided in two sections, each beingin engagement with respective side parts, and the other of the ramps(64,66) is in engagement with the central part. The angle of the ramps(64, 66) with reference to a longitudinal axis of the driving pulleypreferably decreases towards the exterior. A damper material, forinstance a plastic composite that avoids deformation of the ramps, mayfurther cover the parts of the flyweights. It should be noted that thepositive flyweights (62) may be constructed differently. For instance,they may be designed to slide on the ramps (64, 66) instead of rollingthereon.

The flyweights (62) are radially movable between their respective pairof ramps (64, 66). They are forced radially outwards by the centrifugaleffect of the flyweights (62) as they act on the ramps (64, 66),creating a first force biasing the second flange (36) towards the firstflange (28) in function of the rotation speed of the driving pulley(10). The first force tends to increases the winding diameter of thedrivebelt-receiving groove (12), thus the ratio of the CVT. The axialreaction of the flyweights (62) is counterbalanced by the force exertedby a first spring, which is preferably a helical spring (70) coaxiallymounted around the main shaft (16) and pre-loaded in compression. InFIGS. 1, 2 and 5, the spring (70) is set between the positive assemblycarriage (52) and a negative assembly (80), as explained later. Thefirst spring (70) can also be a conical spring (not shown) locatedbetween the positive carriage (52) and a fixed location, such as theintermediary part (74). Other arrangements are also possible, asapparent to a person skilled in the art.

In use, at a stable speed, an equilibrium is reached between the forcestending to close the driving pulley (10), coming from the flyweights(62) of the positive assembly (50), and the forces tending to open thedriving pulley (10), coming from the first spring (70) and the axialreaction of the drivebelt (14) of the flanges (28,36) of the drivingpulley (10).

The present invention is characterized in that the driving pulley (10)further comprises a negative assembly (80). The purpose of the negativeassembly (80) is to generate a second force which is opposite the firstforce generated by the positive assembly (50). The second force will infact increase the rotation speed at which the ratio changes compared toa similar driving pulley (10) without a negative assembly (80). Thesecond force is mainly effective against the first force during anacceleration. However, depending on the design, its effect can be usefulin other situations.

The construction of the negative assembly (80) is similar to that of thepositive assembly (50). The negative assembly (80) comprises a negativecarriage (82) that is slidably mounted around the main shaft (16),preferably by means of bushings (84) or a linear bearing (not shown).The negative assembly (80) also comprises a plurality of pairs offlyweight ramps (94, 96) symmetrically disposed with reference to themain shaft (16). Both ramps (94, 96) of a pair are radially convergingwith reference to the main shaft (16). The first ramps (94) are providedon the end plate (20) and the second ramps (96) are provided on thenegative carriage (82). A flyweight (92), preferably constructed likethe flyweights (62) of the positive assembly (50), is disposed betweeneach pair of flyweight ramps (94, 96). Each flyweight (92) of thenegative assembly (80) is substantially radially movable between arespective pair of ramps (94, 96).

In use, the flyweights (92) are forced radially outwards by thecentrifugal effect and act on the ramps (94, 96), causing an axialreaction that tends to move the negative carriage (82) towards thepositive carriage (52). A second spring (72), preferably a conicalspring highly preloaded in compression, is provided to bias the negativecarriage (82) away from the backside of first flange (28). One side ofthe second spring (72) abuts on an annular part (74) rigidly connectedinside the cylindrical body (22) of the drum (18). The negative carriage(80) will then only start to move from its original position if theaxial reaction force, coming from the flyweights (92) under the effectof the centrifugal force, is higher than the initial preload force ofthe second spring (72). This happens at a given rotation speed of thedriving pulley (10). In the illustrated embodiment, the first spring(70) also applies a force which is against the axial reaction generatedby the flyweights (92).

Like the positive carriage (52), and as best shown in FIGS. 4 and 5, thenegative carriage (82) is operatively connected to the drum (18) bymeans of a plurality of cam followers (86) symmetrically disposed withreference to the main shaft (16). The cam followers (86) are preferablythree rollers or, alternatively, slider buttons (not shown). Each roller(86) is preferably bushing or bearing mounted around a respectivespindle (88) and is guided by a longitudinally-extending slot (90)located in the drum (18). The rollers (86) are freely longitudinallymovable inside their respective slot (90) and the length of the slots(90) substantially corresponds to the displacement of the negativecarriage (82). The rollers (86) and the slots (90) allow the negativecarriage (82) to follow the movement of the drum (18). It should benoted that the number of rollers (86) can be less, especially sincethere is no driving torque going through them. It should also be notedthat the rollers (86) or any other kind of cam followers can be replacedby a linear bearing (not shown) set between the negative carriage (82)and the main shaft (16).

The behavior of the driving pulley (10) is not totally dependant on therotation speed. In fact, the preferred embodiment indirectly uses thedriven pulley of the CVT to control the special features of the drivingpulley (10). As aforesaid, a conventional driven pulley is atorque-sensitive mechanical device. If the load increases, such as whenthe torque from the motor increases, the distance between the flanges ofthe driven pulley will tend to decrease in order to downshift the CVT.The downshift happens if the axial reaction of the drivebelt (14) on theflanges of the driven pulley is stronger than that of the flyweights(62) of the positive assembly (50). If this is the case, the drivenpulley will offset the drivebelt (14) when the ratio is above theminimum ratio, thus forcing the distance between the flanges (28,36) ofthe driving pulley (10) to increase. If the CVT is already at theminimum ratio, then the drivebelt (14) will not be offset but thetension therein will be very high.

The negative assembly (80) is designed to get a more efficient responsefrom the CVT. This is due to the fact that the negative carriage (82) ofthe negative assembly (80) is movable within a range of positions whichoverlaps the range of positions of the positive carriage (52). When therotation speed of the driving pulley (10) is above the threshold valuedictated by the preload of the second spring (72), and optionally by thepreload in the first spring (70) that depends on the relative positionbetween the positive carriage (52) and the negative carriage (82), thenegative carriage (82) moves closer to the positive carriage (52). Thereis an engagement between the positive carriage (52) and the negativecarriage (82) if the rotation speed of the driving pulley (10) is highenough, depending on the ratio. If the ratio is low, the rotation speedhas to be quite high for an engagement. However, if the ratio is high,the positive carriage (52) is already close to the negative assembly(82). During an engagement, the axial reaction of the flyweights (62) ofthe positive carriage (52) is reduced by the axial reaction of theflyweights (92) of the negative assembly (80), minus the force of thesecond spring (72) which increases as it is further compressed. Ofcourse, the threshold rotation speed at which the negative carriage (82)starts to move is below the rotation speed at which the positiveassembly (50) would normally start upshifting the CVT.

The following is an example a vehicle having the mechanicalcharacteristics of a typical automotive application and which was usedto conduct experiments on a driving pulley incorporating the presentinvention. The some of the results of these experiments are shown inFIG. 6 to better illustrate the advantages of the present invention.

EXAMPLE

Motor: 55 HP @ 5000 rpm. Max. design speed of the vehicle: 160 km/hDriving pulley outside diameter: 164 mm Center distance between drivingand driven pulleys: 170 mm Belt pitch length: 710 mm Minimum ratio(underdrive): 0.4 Maximum ratio (overdrive): 2.0 Flyweights (positiveassembly): 3 × 320 gr. Flyweights (negative assembly): 3 × 175 gr. Rampangles (with respect to the longitudinal axis of the main shaft): Ratio:0.4 1.0 2.0 Ramps (positive assembly) 80° 60° 55° Ramps (negativeassembly) — 55° 70° Spring: Preload Rate First spring (70)  35 kg 10kg/cm Second spring (72) 135 kg 30 kg/cm

In a conventional driving pulley, a sustained increase of the torquefrom the motor eventually increases its rotation speed, thus increasesthe rotation speed of the driving pulley (10) as well as the axialreaction from the flyweights (62) of the positive assembly (50). Asaforesaid, a conventional centrifugal system soon becomes proportionallystronger than the cam system of the driven pulley and usually upshiftsthe ratio too early towards the maximum ratio. This is clearly visiblefrom the experimental curve 1 in FIG. 6, where the driving pulley didnot have a negative assembly (80). In that example, the transitionoccured near 2500 RPM and at a vehicle speed of about 20 km/h. Within afew seconds, the upshift of the ratio increased the load on the motorand held the rotation speed of the driving pulley (10) at around 3000RPM. The vehicle continued to accelerate but the ratio of the CVTchanged proportionally until it reached the maximum ratio at 110 km/h.The CVT then became a one-speed transmission up to the maximum speed ofslightly under 140 km/h.

The experimental curve 2 in FIG. 6 shows an example of the relationshipbetween the vehicle speed and the rotation speed of the driving pulley(10) that is provided with a negative assembly (80) according to thepresent invention. In this case, the rotation speed increased to about3250 RPM before the ratio of the CVT changed. The effect of the negativecarriage (82) in the example begins at the minimum ratio. Because itdecreases the effect of the positive carriage (52), it tends to createan overshoot RPM. Then, at a ratio of about 0.8, the relationshipbetween the vehicle speed and the rotation speed of the driving pulley(10) became substantially linear. The change in the curve mainlyoccurred because the angles in the sets of ramps decrease towards theperiphery of the driving pulley (10). The engagement between thepositive carriage (52) and the negative carriage (82) continued all theway to the maximum vehicle speed of 160 km/h. This maximum speed wasover 20 km/h higher than in the previous example because an internalcombustion engine motor is allowed to deliver more power at a higherrotation speed. The motor in the experiment was allowed to reach suchhigh rotation speeds since the ratio was lower than in the exampleillustrated by curve 1. For instance, at 110 km/h, the ratio reached 2.0in curve 1 while it was 1.2 in curve 2. At the maximum speed in curve 1,which is about 137 km/h, the rotation speed was about 3500 RPM, arotation speed at which the motor could not generate enough power underthe given conditions to reach a higher speed. In curve 2, at 137 km/h,the rotation speed of the motor was about 4750 RPM. The increased powercoming out of the motor allowed the vehicle to reach the speed of 160km/h and the motor to reach a rotation speed of over 5000 RPM.

In the experimental curve 3 in FIG. 6, the driving pulley (10) includeda negative assembly (80) in which the range of positions of the negativecarriage (82) was limited by a set of stoppers (78), which are shown inFIG. 1. The stoppers (78) were mounted on the negative carriage (82) andabutted on the intermediary part (74). Normally, and depending on thedesign, the outer portion of the negative carriage (82) can be used tolimit the range of positions when it abuts on the intermediary part (74)or any other part fixed to the drum (18). The stoppers (78) reduced themovement of the negative carriage (82) by a distance d1. This allowed tomoderate the initial effect of the negative assembly (80) (reducing theovershoot effect) and to allow the CVT to upshift from a lower rotationspeed, thereby keeping a low rotation speed of the motor at relativelylow vehicle speeds. The increase of the ratio moved the positivecarriage (52) towards the negative carriage (82). The negative assembly(80) became effective for the rest of the acceleration as soon as therewas an engagement between the positive carriage (52) and the negativecarriage (82). Once an acceleration is over and the speed of the vehicleis stable, the CVT finds a new equilibrium and the rotation speed of themotor tends to decrease due to the change in the torque applied to thedriven pulley.

The reference numeral 4 in FIG. 6 refers to the point in the graph whenthe vehicle speed was stable at 70 km/h. At that point, the ratio is 1.5and the rotation speed of the driving pulley (10) is about 2200 RPM.Then, during a sustained and intense acceleration from that vehiclespeed, the relationship between the vehicle speed and the rotation speedof the driving pulley (10) followed the experimental curve 5. Initially,the rotation speed proportionally rose faster than the increase of thevehicle speed. This is due to the fact that the CVT downshifted to about1.2 within a few seconds. The downshifts occurred because the highertorque on the driven pulley urged the ratio to decrease. As it did, therotation speed of the motor was allowed to increase. The increase of therotation speed allowed the negative carriage (82) to be moved towardsthe positive carriage (52) and eventually reach it to help thedownshift. The second part of this acceleration was identical to that ofthe experimental curve 3.

It should be mentioned that in the example, the driving pulley (10) isdirectly connected to the output shaft of the motor, as it is the casein many applications. Alternatively, it is possible to provide a gearbox or another similar arrangement between the output shaft of the motorand the driving pulley (10). Yet, the presence of the first spring (70)between the positive carriage (52) and the negative carriage (82) allowsthe negative assembly (80) to have an indirect effect on the positiveassembly (50), even when both are not in engagement.

Depending on the design and the conditions, an engagement between thepositive carriage (52) and the negative carriage (82) may not happen ifthe vehicle accelerate at a slow rate. For instance, if only a smalltorque is applied on the driven pulley, the positive assembly (50) isalmost free to move as soon as the rotation speed changes. The rotationspeed could be kept lower than the threshold value where the negativeassembly (80) comes into action.

The present invention is not limited to the described embodiments andencompasses any alternative embodiments within the limits defined by theclaims.

1. A driving pulley for a continuously variable transmission, thedriving pulley being coaxially mountable around a main shaft androtatable at a variable rotation speed, the driving pulley comprising: afirst flange having opposite first and second sides, the first sidebeing provided with a conical wall; a second flange coaxial with thefirst flange and having a conical wall facing the conical wall of thefirst flange to form a drivebelt-receiving groove around which adrivebelt is wound, the second flange being axially movable withreference to the first flange; first means for connecting the firstflange to the main shaft in a torque-transmitting engagement; secondmeans for connecting the second flange to the main shaft in atorque-transmitting engagement; a positive assembly comprising: apositive carriage coaxial with the first flange and rigidly connected tothe second flange; third means for connecting the positive carriage tothe main shaft in a torque-transmitting engagement; at least twosymmetrically-disposed pairs of radially-converging andmutually-opposite first ramps, each pair having one ramp connected tothe positive carriage and another ramp connected to the second side ofthe first flange; and radially-movable flyweights, each set between acorresponding pair of first ramps; fourth means for generating a returnforce urging the second flange to move away from the first flange; anegative assembly comprising: a negative carriage coaxial and axiallymovable with reference to the first flange, the negative carriage beingconfigured and disposed to be in engagement with the positive carriage;fifth means for connecting the negative carriage to the main shaft in atorque-transmitting engagement; at least two symmetrically-disposedpairs of radially-converging and mutually-opposite second ramps, eachpair having one ramp connected to the negative carriage and another rampconnected to an end plate fixed with reference to the first flange;sixth means for connecting the end plate to the main shaft in atorque-transmitting engagement; and radially-movable flyweights, eachset between a corresponding pair of second ramps; and seventh means forgenerating a return force urging the negative carriage away from thefirst flange.
 2. A driving pulley according to claim 1, wherein thefirst means comprise a hollow drum coaxially disposed around the mainshaft, the drum having one end rigidly connectable to the main shaft anda second end rigidly connected to the second side of the first flange.3. A driving pulley according to claim 2, wherein the third meanscomprise a plurality of pairs of radially-projecting cam followers andcorresponding axially-extending slots, each pair having one among thecam follower and the slot located on the positive carriage and havingthe other located on the drum.
 4. A driving pulley according to claim 3,wherein the cam followers are rollers.
 5. A driving pulley according toclaim 1, wherein the second means comprise an axially slidable sleevecoaxially mounted around the main shaft, the sleeve rigidly connectingtogether the second flange and the positive carriage.
 6. A drivingpulley according to claim 1, wherein the fourth means comprise a springmounted between the positive carriage and the negative carriage.
 7. Adriving pulley according to claim 1, wherein the fifth means comprise aplurality of pairs of radially-projecting cam followers andcorresponding axially-extending slots, each pair having one among thecam follower and the slot located on the negative carriage and havingthe other located on the drum.
 8. A driving pulley according to claim 7,wherein the cam followers are rollers.
 9. A driving pulley according toclaim 2, wherein the end plate is a portion of the drum.
 10. A drivingpulley according to claim 9, wherein the seventh means comprise a springhaving one end connected to the drum and other end connected to thenegative assembly.
 11. A driving pulley according to claim 1, furthercomprising means for limiting the movement of the negative carriage withreference to the positive carriage.
 12. A driving pulley according toclaim 11, wherein the means for limiting the movement of the negativecarriage comprise at least one stopper set between an upper part of thenegative carriage and a location fixed with reference to the firstflange.